How to align all vertices so that they would be lying on one plane?
Couple of ways you could do this:

Method 1


Select your face
Change the 'Transform Orientation' to 'Normal'

Hit 'S' to scale
Double-hit 'Z' to scale along the face's normal
Type 0
Hit enter


Method 2


Add a new plane (single face)
Rotate it as desired to define your belonged normal axis
Again change your Transformation Orientation to Normal
Create your custom orientation in the right-3dview-panel

Rename it and use it in the same way as 'Method 1'